1. Field of the Invention
An embodiment described herein relates generally to a controller for a power converter that drives a motor.
2. Description of the Related Art
In general, a regeneration brake is known as means for decelerating a motor. When the regeneration brake is used, it must consume the regeneration energy generated by the motor. As a method of consuming the regeneration energy, there is a method of, for example, providing a resistance on the DC-side of an inverter for consuming the energy. In this case, it is necessary to add a resistor as new hardware. There is another method, in which the energy is returned to the power supply side. In this method, a converter for supplying DC power to the inverter that drives the motor is required to perform inversion, and hence it is necessary to replace a diode rectifier with an expensive converter than that. In light of these requirements, such methods as below utilizing control by an inverter have been proposed.
A first method is a method of strengthening, utilizing indirect field oriented control (IFOC), a magnetic flux within a range in which the DC capacitor voltage does not exceed the upper limit, thereby causing a motor to consume energy (Marko Hinkkanen, et al., “Braking Scheme for Vector-Controlled Induction Motor Drives Equipped With Diode Rectifier Without Braking Resistor,” IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, IEEE, September/October 2006, VOL. 42, No. 5, pp. 1257-1263).
A second method is a method of superimposing different frequencies, and is also a braking method utilizing the above-mentioned IFOC. In this method, a relatively low-order frequency current is superimposed on an output current to cause a motor to consume energy (Mukul Rastogi, et al., “Dual-Frequency Braking in AC Drives,” IEEE TRANSACTIONS ON POWER ELECTRONICS, IEEE, 2002, November, VOL. 17, No. 6, pp. 1032-1040).
A third method is a high-frequency wave superimposing method, and is also a braking method utilizing the above-mentioned IFOC. In this method, a current containing a large number of high-frequency components is superimposed on an output current to thereby cause a motor to consume energy (Jinsheng Jiang, et al., “An Efficient Braking Method for Controlled AC Drives With a Diode Rectifier Front End,” IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, IEEE, 2001, September/October, VOL. 37, No. 5, pp. 1299-1307).
IFOC is a motor control method known as general vector control. However, the above-mentioned methods principally have the following problems. Since IFOC is a method of controlling current to indirectly control flux, and the first-order lag transfer function determined from a motor constant exists between the current and the flux. Accordingly, a motor response beyond the first order lag cannot be expected. Further, in the second and third methods, frequencies other than the inverter output frequency are superimposed, which principally causes torque ripple.
In contrast to the above three methods, there is a direct torque control method (fourth method). The fourth method is a method of directly controlling torque and flux to increase the magnitude of the stator flux with a fast response so as to make a motor consume energy. In this control, since the flux can be directly controlled, there is no influence of the first order lag. Therefore, the fourth method exhibits a high response compared to IFOC. In the fourth method, however, in order to determine a switching pattern on the inverter for driving the motor, hysteresis must be provided for the torque and flux control, which principally causes torque ripple (Cristian Lascu, et al., “A Modified Direct Torque Control for Induction, Motor Sensorless Drive,” IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, IEEE, January/February 2000, VOL. 36, No. 1, pp. 122-130).